One of the most important applications for crystal growing is in the area of semiconductor substrate material. This substrate material is available in ribbon form in which ribbons of large-grain polycrystalline or single crystalline material are grown. The ribbons usually of silicon, have application in the semiconductor industry and are especially well adapted for use as solar cell substrates.
In U.S. patent application Ser. No. 439,603, now U.S. Pat. No. 4,627,887 a continuation of U.S. patent application Ser. No. 214,741 filed Dec. 11, 1980 now abandoned which is a continuation-in-part of U.S. patent application Ser. No. 109,865 filed Jan. 7, 1980 and which U.S. patent application Ser. No. 439,603 has been allowed on May 21, 1986, there is a detailed discussion of the prior art methods and apparatus for the fabrication of crystalline ribbon from a melt. Also included in application Ser. No. 439,603, and now U.S. Pat. No. 4,627,887 are discussions of the problems associated with the prior art methods and apparatus. These discussion are incorporated herein by reference thereto. U.S. patent application Ser. No. 591,284 entitled COMPOSITE STRINGS AND METHODS FOR MAKING SAME, filed Mar. 19, 1984 and U.S. patent application Ser. No. 695,492 entitled METHODS AND APPARATUS FOR MINORITY CARRIER DIFFUSION LENGTH MEASUREMENT, filed Jan. 28, 1985 both applications of which are pending and are drawn toward string stabilized pulling.
In String Stabilized Growth (SSG) a ribbon with an unobstructed central web is grown directly from the melt. The edge positions are determined and stabilized by wetted strings, strands or filaments running continuously through the melt and "frozen" into the growing ribbon. These strands are of a material different from the melt. In the procesds (SSG) a surface tension controlled melt liquid meniscus is defined on the bottom by the melt surface, on the top by a growth interface, and at the edges by the wetted "frozen-in" strings. The growth interface is defined by the growing crystal and the melt liquid meniscus and is existent when the crystal is being grown. A freezing boundary is also defined and is similar to the growth interface except that is exists prior to the growing of the crystal ribbon. That is the freezing boundary is that boundary comprising the upper boundary of the melt liquid meniscus which freezes onto the seed. When the crystal ribbon is being grown the growth interface and the freezing boundary are essentially the same physical location on the growing ribbon.
The invention provides for new and improved methods for seeding or initiating the growth of string stabilized ribbon grown according to the methods and apparatus disclosed in application Ser. No. 439,603, now U.S. Pat. No. 4,627,887.